This invention relates to a method and an apparatus for converting a line density of a bi-level or binary image signal.
According to the G3 standards of facsimile communication, the resolution in the main scanning direction is fixed to 8.037 pixel/mm while the resolution in the sub scanning direction can be changed among 3.85 pixel/mm, 7.7 pixel/mm, and 15.4 pixel/mm in dependence upon the contents of a transmitted image.
Some high-resolution printers such as laser printers can print out information at a density equal to or above 16 pixel/mm. In cases where such high-resolution printers are used in combination with a G3 facsimile system, it is necessary to provide a device for converting a low-density image signal into a high-density image signal. Such a line-density converting device is necessary also in other systems, for example, an image data file system.
Various methods of converting the line density of a bi-level image signal have been proposed. Improvements are desired in these prior art methods.